I've got the same specs in my 78 KE250 manual:
419.8 feet at 9.46 seconds low speed
1186.2 feet at 12.6 seconds high speed

I would find top gear 50-80 specs more useful, like they do in automotive tests. These passing tests are a good indicator of top end power, I'm assuming they start the pull from the lowest gear available for best acceleration. My beef with the DT360, other than the poor carburetion and pinging, has always been low-mid power. Doesn't do much unless you wind it out.

I do find it surprising that the KE250 matches the DT360 in the high speed test. Those are rated at 22 HP at 6000 RPM.

Actual crankcase volume.

I've hit a steep part of the learning curve when it comes to crankcase volume. It had never been apparent to me this is a significant factor in the "tuning" of the intake system.

In the 70's, with Gordon Jennings' book in hand, one could take a reasonable stab at creating a tuned exhaust that would alter a 2 stroke's power band.

The concept of "stuffing" the crankcase used to mean (to me and I have no experience with this) plugging the balance holes in the crankshaft. It has obviously evolved into a more refined and involved process.

Is there similar literature about the intake system as there is about the exhaust?

I'm pretty sure Jennings covered "crankcase stuffing". He didn't say much or advocate doing it because by the 1970's two strokes had what would be considered "high" primary (crankcase) compression ratios. Yamaha might even be mentioned specifically as having high primary compression.

The practice of "stuffing" came from the early two strokes... perhaps primarily the BSA Bantam and all the other popular variants of the old DKW design that predated expansion chambers.... they had really low crankcase compression and no pipe to help with gas flow dynamics. So that practice of "stuffing" was actually a carryover and was not needed on newer engines.

My understanding is that when manufacturers got away from trying to "force" the mixture up through the ports with primary compression is when power levels really started to climb. Modern high output two strokes don't have high crankcase compression.

Jennings also covered intake tract tuning in the book.

A great example of a modern high output two stroke would be a case reed 125cc motocross engine. These have stuffed cranks and no more case volume than is necessary to achieve the desired cross sectional area through the transfer tunnels. In a modern 250cc motocross engine they not only stuff the crank but even go as far as to install welsh plugs in the crank pin to isolate the hollow core from the crankcase volume. They also mount the reed cage as close to the bore center line as possible, which Yamaha clearly did a poor job of in the DT360 motor. Without exception, every modern high performance two stroke engine utilizes crank stuffing and minimal crankcase volume. This is beneficial for making power and critical for clean carburetion.

In that case reed 125cc engine, a little more than half of the total power output is generated by the pipe. 250cc and larger engines rely less on the pipe and more on primary compression to make a good spread of power at lower RPM. The DT360 of course has very little pipe effect, lots of back pressure and very large case volume. It makes little power until you get the revs up, at which point it makes average power. The biggest drawback is certainly the driveability, for which there is no excuse in such a mildly tuned engine. We can't blame the carb, the porting, the pipe, the ignition or the head. These are all comparable to the earlier and later Yamaha enduros. The one thing that the 360 has the other do not is a lot of room in the basement.

Gordon Jennings and Graham Bell wrote these books about 50 years ago. They have a lot of good information but they are far from the current state of the art. Crankcase volume and primary compression are a critical part of the two stroke engine. You can have too much, you can have too little. In a motor like the DT360 where you could smuggle contraband in the transfer port tunnels, I think we are on the too large side of case volume. It certainly runs better with some filler in there.

I've never found much. Generally the manufacturer has done this properly. You won't find a high performance two stroke made in the last few decades that doesn't have aluminum or phenolic stuffers built into the crank. They all do.

What we can't derive from research we can learn from observation. Modern two strokes do not leave dead space anywhere in the crankcase or intake tract. We have stuffers in the reed block too, less volume ahead of the carb means better intake signal. Cleaner carburetion, better throttle response.

I would describe my DT360 as just the opposite of what some of you are reporting...
My bike delivers tractor like torque from 2000rpm and up, hitting a wall at 6500-7000 because of my pipes tuned length.
It also cruises nice and civilized at 1/8 throttle, 30mph and higher. a steady speed of 25and below can be a bit hard to achieve.

Here is a short video I took of riding my bike around my neighborhood and in a bit of traffic.
steady state 30mph cruise is at around 6 minutes in the video, sorry about the wind noise, its my first video

I'm happy with its low speed performance, but there is no top end to speak of, just a strong midrange to 6.5k then its done.

The large crankcase area doesnt lend itself well to the factory configuration of low exhaust port timing and no expansion chamber, but in my eyes the way to fix that is to make power with a pipe,and adding more blowdown exhaust duration. This makes it more powerful and more like a modern 2 stroke. Reducing crankcase volume to improve throttle response and low speed running may be the thing to do to work with the factory muffler and port timing, but to me that is less efficient.

I feel like my bike runs smooth at low rpm with the factory crankcase volume, let me know what you guys think!

That pipe sounds so good. It certainly helps draw mixture through the transfers with the long, tapered head pipe. If I remember correctly you have lifted the cylinder 2mm as well, and milled the top by the same amount?

I'd still fill the transfers on it. I'm running the MX360 pipe which is probably about half as effective as the one you've constructed and it works well with the stuffed cylinder. The MX pipe does nothing beneficial until about 5000 RPM. I rarely wind it up that high though. I can get everywhere I need to go and cruise up to 55 without exceeding 4500 RPM.

Wind noise is a problem with the helmet mounted camera. Last video I got while cruising around is pretty lame:



The silencer is off of a late model KX or YZ250 and it is very effective. The carburetion is quite good but you can't hear anything but the wind. Of course you can't really tell anything about throttle response or bottom end torque either.

I watched your earlier video on your "Scrapper Bike" topic and was impressed at how civilized your bike ran while riding around town. The pipe does not sound overly loud, either. Nicely done!

Are you running the stock air box and OEM filter?

One of my DT360s has an air box that was modified by the previous owner. A pattern of holes was added to the front of the box.

I have covered them to see if there was a difference in how the engine ran. There is a marked negative impact at wide open throttle, the higher gears and higher RPMS with the holes covered. The engine pulls higher into the RPM range with the holes open. This bike has a Sleeper Pipe on it.

Akara1 - that’s one sweet motor!

I definitely would appreciate understanding your intake and exhaust set up. Also if you have stock jetting in carb, stock CDI?

I had a piston meltdown and after a 60 mph cruise in 4th, never found root cause. Gas was flowing fast to rebuilt carb and main jet was clear and seals and gaskets not leaking air.

I have completely stripped mine down, crank and all running gear perfect. So I am going to put in new bearings, seals and a fresh 1st OS bore, hermetically sealing it all up.

Hate melt this thing down again, really want to restore this childhood dream so any inputs appreciated

Don't want to hijack this thread so replying in your "running perfect" thread.